Classifications

• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
  • C23C8/60—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using solids, e.g. powders, pastes
  • C23C8/62—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using solids, e.g. powders, pastes only one element being applied
  • C23C8/64—Carburising
  • C23C8/66—Carburising of ferrous surfaces
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/06—Surface hardening
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/002—Heat treatment of ferrous alloys containing Cr
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
  • C23C8/02—Pretreatment of the material to be coated
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
  • C23C8/04—Treatment of selected surface areas, e.g. using masks
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
  • C23C8/06—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases
  • C23C8/08—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases only one element being applied
  • C23C8/20—Carburising
  • C23C8/22—Carburising of ferrous surfaces
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
  • C23C8/80—After-treatment
• • E—FIXED CONSTRUCTIONS
  • E21—EARTH OR ROCK DRILLING; MINING
  • E21B—EARTH OR ROCK DRILLING; OBTAINING OIL, GAS, WATER, SOLUBLE OR MELTABLE MATERIALS OR A SLURRY OF MINERALS FROM WELLS
  • E21B10/00—Drill bits
  • E21B10/46—Drill bits characterised by wear resisting parts, e.g. diamond inserts
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2241/00—Treatments in a special environment

Definitions

• the invention relates to the steel industry, and more particularly to the grades of case-hardening steels having a high resilience.
• Such a drill bit is a forged tool made up of three rotating steel cones "entangled" one inside the other and making it possible to grind the geological formations during oil or gas exploration operations. These three cones are in rotation, via one or more bearings, on three steel arms assembled by welding.
• the raceways machined on the arms and inside the cones are generally, in conventional production processes, treated superficially by atmospheric case hardening to reach a conventional depth, where the Vickers hardness is 550 HV, ranging on average between 1 and 1.5 mm.
• the present invention relates to a new steel grade which can be used for the manufacture of cones and arms.
• drill bits There are several kinds of such drill bits.
• One of them is the "inserted tooth” drill bit, in which pins, most often made of tungsten carbide obtained by powder metallurgy, are set in each of the cones.
• the steel which is the subject of the present invention is not limited, in its use, to this type of drill bits, and could also be used for the production of drill bits with "machined teeth”.
• the document US-A-6 146 472 presents an example.
• the increase in resilience is obtained there by controlling the resistance to magnification of the austenitic grain via the use of an Nb-Al-N microalloy, associated with an optimized heat treatment.
• the resilience values indicated in this document are at best close to 60J, and the examples presented are castings which do not make it possible to verify the hardenability criterion ⁇ ⁇ 3.5 HRC.
• WO 03/062484 describes a steel for drill bits.
• the object of the invention is to provide a case hardening steel which can be used in particular for the manufacture of drill bits, requiring no addition of Ni and nevertheless meeting all the criteria of ductility, hardenability, Re, Rm and Kv mentioned above.
• the invention is based on a careful adjustment of the composition of the steel, making it possible to satisfy all the criteria mentioned above.
• the steel object of the present invention also differs from that described in US-A-6 146 472 in that the accessible resiliences are significantly higher, and in that the improvement in resilience is not generated, at least mainly, by a control of the grain size.
• This has the advantage of not modifying the suitability of the shade for thermomechanical treatment and of limiting the risk of abnormal enlargement of the austenitic grain during cementation.
• the segregating effect of niobium which could lead to a heterogeneous austenitic grain size is avoided.
• the level of resilience accessible by the present invention is also significantly higher.
• case hardening used with the steel described in the present invention is not limited to the atmospheric hardening process which could be replaced by other surface hardening processes, for example low pressure hardening.
• the present invention is based on a steel whose composition is defined below. All contents are given in weight percentages.
• a composition defined as described below it is possible to develop, without voluntary addition of nickel and without using significant amounts of other expensive elements, a steel having a hardenability, mechanical characteristics after hardening followed by a tempering and carburizing ability (carbon uptake, core resilience, carburizing depth, residual austenite content, etc.) close to those of the reference grades of 3.5% Ni usually used for the manufacture of drill bits drilling.
• the C content is between 0.10% and 0.15%, ie a carbon content limited to a relatively narrow range, and which is low, compared to those generally encountered in case hardening steels. This low carbon content makes it possible to achieve very high resilience in the core of cemented parts.
• the loss of hardenability and the reduction in the hardness at the heart of the products after cementation, which would normally result from this reduction in the C content, are compensated by an optimized adjustment of the concentration of the other alloying elements.
• the Mn content is between 0.8% and 2%.
• Manganese is used with chromium and molybdenum to compensate for the loss of hardenability associated with the decrease in carbon content. For its effect to be sufficient, a content greater than or equal to 0.8% is required. Since this alloying element can pose segregation problems, it is preferable that its concentration does not exceed 2%.
• the Cr content is between 1% and 2.5%.
• chromium is used to ensure a sufficient level of hardenability in the shade.
• the minimum content of 1% is chosen so that the effect of this alloying element on the hardenability is sufficient.
• the maximum content of 2.5% is defined so as to avoid an adverse effect on the properties of use, in particular by the formation of coarse chromium carbides.
• the Mo content is between 0.2% and 0.6%.
• Molybdenum is a third element used to adjust the hardenability of the shade. It is also an alloying element which can be judiciously used to increase the resilience, in particular at low temperature. Molybdenum also makes it possible to exacerbate the effect of boron on the hardenability, and can therefore be used for this purpose in the case of grades alloyed with boron. For a content of less than 0.2% the increase in the hardenability is too small and this value is therefore chosen as the minimum level. For high concentrations, molybdenum tends to reduce the aptitude of steels for forging. In addition, since it is an expensive alloying element, its use at an excessive content would lead to a loss of the economic benefit brought by the non-use of nickel. For these reasons, a maximum content of 0.6% is preferred.
• the Si content is less than 0.35%.
• silicon can be used as a deoxidation element.
• the residual content of this element generally does not exceed 0.35% anyway. It is also advisable not to exceed a content of 0.35% in the steels of the invention, since silicon is an alloying element which can limit, by barrier effect, the carbon uptake during cementation.
• the Ni content is less than or equal to 0.7%, preferably 0.3%.
• one of the objects of the present invention is to make it possible to dispense with an addition voluntary of this element. However, it is always present in the residual state in the raw materials used to make steel, especially in scrap.
• the content of 0.3% corresponds to the maximum content most generally encountered when no voluntary addition of nickel is made during production.
• the B content is less than 0.005%.
• Boron is an optional element. It can be used to optimally adjust the hardenability of the grade if the contents of Mn, Cr and Mo are not quite sufficient for this purpose. But for this alloying element to act effectively on quenchability, it must be kept in solid solution. For this, precipitation of nitrides or boron oxides must be avoided. This can be achieved by adding an alloying element with higher affinity to nitrogen, for example titanium, and by controlling the production process to limit the dissolution of nitrogen and oxygen in the steel.
• the Ti content is less than 0.1% and preferably less than 0.04%.
• the titanium is optionally added to allow the boron to be kept in solid solution by precipitation of titanium nitrides which reduce the amount of nitrogen which would be capable of combining with the boron. Its content should optimally be chosen according to the amount of nitrogen in the grade.
• a stoichiometric amount of titanium must be added to ensure precipitation of all the nitrogen contained in the steel in the form of TiN, and thus maintain the boron in solid solution. This is verified if the Ti / N ratio is greater than 3.4.
• the effect of boron on the hardenability can still be expressed but is less marked. Beyond the prescribed limit, there is a risk of formation of too coarse TiN during solidification, and moreover the addition of Ti becomes excessively expensive.
• the N content is less than 0.02%, preferably less than 0.01%.
• boron and titanium it is necessary to control the nitrogen content of the steel to limit the risk of formation of coarse titanium nitrides TiN, which can deteriorate the properties of use of the product.
• boron 5 to 50 ppm
• a nitrogen content of less than 0.01% is therefore recommended. If boron is not used (B ⁇ 5 ppm), it is not absolutely essential to strictly control the nitrogen content, which can then go up to 0.02% without any harmful effect on the properties of the steel produced. .
• Aluminum is an optional element. It can be used as a deoxidizer for steel to replace silicon, and to optimize the resistance of the austenitic grain during cementation.
• V content is at most 0.3%.
• Vanadium is an optional element. It can be used as a micro-alloy element for better control of the grain size during cementation, bringing an additional improvement in resilience.
• the P content is at most 0.025%. This limit is recommended so as not to risk weakening the steel. Too high a content, this element tends to segregate at the austenitic grain boundaries, which can lead to an increase in the ductile-brittle transition temperature and to a reduction in the resilience at room temperature.
• the Cu content is at most 1%, preferably at most 0.6%.
• a maximum content of 1% is recommended because it is an expensive element which does not provide any benefit of hardenability or resilience.
• the preferred maximum value of 0.6% is a content usually recognized as being below which copper has no significant effect on the mechanical properties of steel. However, its use at a higher content is possible without modifying the suitability of the grade to be used for the manufacture of drill bits.
• the content of S is not strictly imposed in the definition of the steel according to the most general invention, but it must be controlled according to the envisaged application.
• a low content will be sought if one wishes to improve the inclusiveness by not forming sulphides (typically ⁇ 0.01%) and a higher content may be chosen (typically from 0.03% to 0.1%) if a gain in machinability is sought after and subject to the inclusion cleanliness remaining in conformity with the requirements required by the envisaged application for steel.
• the O content is most often at most 0.003% (30 ppm), so as to optimize the inclusiveness. This limit may possibly be exceeded if the future application of the steel does not require very good inclusion cleanliness, and in any case a determined O content does not constitute an intrinsic property of the steel according to the invention.
• low pressure carburizing can be used to avoid possible problems of surface and / or intergranular oxidation during treatment, and also to access greater carburizing depths than the 1 to 2 mm usually accessible by atmospheric carburizing and / or reducing the carburizing time thanks to the high temperature at which the low pressure carburizing is practiced.
• the mechanical properties obtained on the final product depend not only on the composition of the steel, but also on the thermal and thermomechanical treatments which it undergoes until the product is obtained. It can however be noted that in the case where the final product must be case-hardened, the conditions for its hot forming by forging, rolling or the like are of little importance. Indeed, the case hardening is accompanied by a quenching and tempering operation which gives the product a new structure and eliminates the consequences of hot forming. It is then this treatment which gives the product most of its mechanical properties, if it is not itself followed by any other treatment which could modify them.
• the steel Before forging, the steel is in the form of ingots of square section 100 ⁇ 100 mm and 200 mm in height. After forging, the 40x40 mm section bars are cooled in still air and then normalized for 2 hours at a temperature of 875, 900 or 925 ° C, chosen according to the transformation point Ac3 of the grade. This standardization is intended to homogenize the carbon content and the initial microstructure throughout the product.
• the composition of the various shades tested is given in Table 1.
• the flows 1 to 4 are those whose composition is in accordance with the present invention.
• Castings Nos. 5 to 10 are those for which at least one of the alloying elements is outside the claimed ranges. All concentrations are given in% by weight, except nitrogen, oxygen and boron which are given in ppm by weight.
• the table also indicates the temperature of the transformation point Ac3 (in ° C) of each of the grades.
• Table 1 compositions and temperatures Ac3 of the samples tested Ech VS % Yes % Mn% Ni% Cr% MB% S% P% O pp m Al% N ppm Cu% B ppm Ti% V% Ac3 ° C Inv.
• the O contents of the various samples are all between 7 and 21 ppm and do not significantly influence the properties obtained.
• the hardenability of the different samples was evaluated by means of Jominy tests.
• the austenitization temperature was chosen, depending on the transformation point of the steel considered, from temperatures 875, 900 and 925 ° C.
• This heat treatment cycle makes it possible to estimate the expected resilience at the heart of the parts treated by case hardening.
• all of the grades whose composition is in accordance with the present invention are characterized by mechanical characteristics which are greater than the minimum required for the production of drill bits, ie Re greater than 900 MPa, Rm greater than 1200 MPa, Kv at 20 ° C. greater at 75 J, and by a quenchability satisfying the criteria ⁇ ⁇ 3.5 HRC and J 1 > 40 HRC.
• all the grades whose composition is outside the present invention have insufficient quenchability and / or mechanical characteristics which are too weak. This is, in particular, the case of sample 6 for which the mechanical characteristics Re and Rm are in any case deeply too weak for the steel to be usable for making drill bits, and for which it was not found it useful to measure resilience.
• a 13NiCrMo13 cylinder was therefore placed in the carburizing charge to serve as a reference and to determine the reference characteristics which the grades produced in accordance with the present invention must achieve for the sample format considered.
• the composition of the casting used as a reference is given in Table 3.
• Table 3 composition of the 13NiCrMo13 steel reference sample VS (%) Yes (%) Mn (%) Ni (%) Cr (%) MB (%) S (%) P (%) O (ppm) Al (%) N (ppm) Cu (%) 0.13 0.23 0.70 3.24 1.44 0.11 0.005 0.01 11 0.028 78 0.19
• the carbon potential in the diffusion phase (diff. Carbon potential) has been adapted to the grade treated so as to control the surface content of residual austenite.

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